A sample receiving light and reflecting the same will generally modify the polarisation thereof.
It is possible to use this property to visualise a sample or to characterise said sample by measuring its ellipsometric parameters designated generally as ψ and Δ.
In this view, one may, for instance, refer to the book by Azzam and Bashara published in 1979.
Initially, it has been sought to process the extinction of the Fresnel coefficient rp at the Brewster angle in order to provide an accurate ellipsometric measurement of the parameters ψ and Δ (ellipsometry) or sensitive display of very thin films, notably at the surface of water (Brewster angle microscopy).
Besides, it has been sought to irradiate a zone of a sample under a single incidence and a single azimuth in order to measure the parameters ψ and Δ corresponding to this zone.
The aim within the framework of this invention is to provide simultaneous processing of the parameters ψ and Δ for a number of points of a sample, each defined by their coordinates x and y. This is called ellipsometric two-dimensional display or measurement of a sample.
Moreover, this invention relates to small samples that may be observed, displayed or measured under an optical reflection microscope. It may be conventional microscopy, microscopy with differential interferential contrast or fluorescence microscopy.
This type of microscopic observations poses particular constraints inasmuch as, on the one hand, the microscope lenses have a wide digital aperture which creates observation conditions significantly different from the usual conditions of the ellipsometric measurements wherein the beams, illuminating beams as well as measuring beams (or reflected beams) are generally small aperture collimated beams and, on the other hand, where the illuminating beams are most often distributed uniformly around the normal incidence, i.e. within a range of angles of incidence hardly lending themselves to ellipsometry.
Still, display methods based on the use of an antiglare substrate have been suggested previously, but they resort to the “incoherent reflectivity” of the substrate. The substrates suggested previously are therefore antiglare for a non-polarised light or for a polarised light with a constant polarisation direction relative to the plane of incidence, which is incompatible with the use of a microscope. The principle is based on the minimisation of the second member of the equation (E4).
                              Φ          N                =                              (                          θ              ,                              N                ⁢                                                                  ⁢                P                                      )                    =                                    1              2                        ⁢                          (                                                                                                              r                      p                                                                            2                                +                                                                                                r                      s                                                                            2                                            )                                                  E4      where rp and rs are the complex reflection coefficients of each polarisation on the substrate affected which depend implicitly on x and on y, ΦN(θ,NP) being the normalised flux reflected for an angle of incidence , in non-polarised light.
It is obvious that complete extinction is only possible for |rp|=|rs|=0, which is an extremely restricting condition, since the values of both Fresnel coefficients are set. The condition of complete extinction, |rp+rs|=0, is far more flexible since it is solely translated into a relation between both Fresnel coefficients,rp=−rs  E6
Antiglare substrates for a polarised light have also been suggested in order to enhance the performances of the ellipsometers, but the ellipsometry and the optical microscopy had been considered until now as incompatible.
The aim of the invention is therefore to provide an ellipsometric two-dimensional display of an object with very small thickness, invisible under optical microscope under observation conditions known as compatible with the use of a commercially available optical microscope.
In spite of this, according to the invention, it is possible simultaneously to visualise the object and to measure its thickness and its index under microscope.